Pipe bursting is a well-known technique for replacing old underground conduit, such as ceramic or metal pipe, without having to dig a trench and remove the conduit to be replaced. (As used herein, unless otherwise noted, the term “pipe” is intended to include all types of conduit typically used in underground installations.) In pipe bursting, a bursting head, sometimes called a mole, is pushed or pulled through the pipe to be replaced to rupture (for example by fracture or by slitting) and displace the old pipe outwardly, while at the same time pulling or pushing a replacement pipe behind it (typically high density polypropylene) to occupy the space left by the displaced old pipe. The bursting head typically includes a tapered, roughly frusto-conical, outer geometry which contacts the pipe in generally coaxial alignment. When the bursting head is moved axially, the outer surface imparts force to the pipe, including a radially outward component, thereby producing a hoop stress in the pipe. When the hoop stress exceeds the tensile strength of the pipe material, the pipe is ruptured. Irregularities, such as cutting blades, may be provided in the outer surface of the bursting head to create regions of higher stress to assist in fracturing or in slitting the old pipe lengthwise.
A particularly useful type of pipe bursting system employs a wire cable to pull the bursting head, wherein the cable is pulled, either intermittently or continuously, with enough force to cause the bursting head to burst the pipe. This process is sometimes referred to as “static” bursting because the load placed upon the pipe by the bursting head, although variable, usually varies little compared to its magnitude during the normal bursting process. To pull the cable, the cable is grasped by one or more grippers, namely, devices which periodically grip and release the cable. The grippers are mounted on and moved by a hydraulically movable structure, such as a so-called bridge, or other suitable movable support such as a rocker arm, or lever. (Although winches can be used for static bursting, the power requirements typically necessitate the use of large and unwieldy equipment. Moreover, the pulling power typically varies as the cable is wound around the drum of the winch, which is undesirable and may require the addition of capstans.) During a recovery stroke, the gripper or grippers are released and returned to their original position for the next pulling stroke. An additional gripper or grippers may be used to hold the cable in a fixed position or to take over the pulling function (i.e. as a continuous puller), while the released gripper is returning to its original position. Thus, in a series of pulling strokes somewhat similar to a person pulling a rope by hand, the bursting head is moved hydraulically in a substantially linear direction. Subsequently, the gripper releases the cable and is returned to its original position (the return stroke) for the next pulling stroke to begin. Examples of this type of bursting system may be found in U.S. Pat. Nos. 6,305,880 and 8,702,349, both of which are assigned to the assignee of the present invention and are incorporated herein by reference.
In the foregoing type of pipe bursting apparatus, the grippers are moved by one or more hydraulic pistons to pull the cable. The force or pulling load imparted to the cable during static pipe bursting can be substantial, up to a hundred tons or even much more depending on factors such as pipe size and material, underground terrain, and cable load capacity. This is particularly true when irregularities are encountered, such as pipe connectors or valve housings, malleable materials which collapse into accordion-like segments, concrete overpours, rocky soil, and the like. Such irregularities can dramatically slow or even stall the pipe bursting process.
In some types of pipe bursting systems, the bursting head or mole is propelled by an impactor, which pushes the mole through the pipe being replaced by repetitive blows of a pneumatic hammer. Thus, the hoop stress required to burst the pipe is created by the force of the blows imparted to the bursting head by the hammer. Many of these types of systems employ a cable and winch to prevent the impactor-driven mole from straying from the pipe being replaced by following a path of least resistance instead of staying within the old pipe. In such systems, a so-called constant tension winch is frequently used to keep the cable-guided bursting head in the required orientation. The tension on the cable in operation of such systems is typically low, well under the recommended maximum operating load designated by the cable manufacturer. (Typically, manufacturers recommend a maximum operating load of 20% of the breaking strength of the cable.) Also, as mentioned above, pulling power in winch type systems is limited, for a given scale of equipment, by the drum strength of the winch and by the fact that the cable often cuts through the spooled layers. As a consequence, there is no true static bursting in such systems. Instead, the bursting is accomplished substantially entirely by the forces imparted by the hammer.
The cables typically used in pipe bursting systems are of conventional twisted wire rope which is available in different diameters and twist directions, depending on the load to be imposed on the cable. Usually, the larger the cable diameter, the greater the load it will sustain. But the greater the diameter, the heavier and stiffer the cable, making it much more difficult to handle the cable at the job site. Moreover, most cable manufacturers recommend that the cable load not exceed twenty percent of the cable's yield strength. It can be inferred that loading cables beyond that level is not desirable.
Another problem with systems relying primarily on a pneumatic hammer to supply the pipe bursting force lies with the power requirements to drive the hammer. To provide the necessary hoop stress to burst the pipe, very large compressors are needed, resulting in higher expense and unwieldiness. Ideally, for typical jobsite pipe bursting, the capacity of the compressor is at or, preferably, below that of the compressors commonly used for hand-held jackhammers. This is usually not adequate for many hammer type bursting requirements.
Static pipe bursting using hydraulic pullers offers the advantage of lighter and lower cost equipment. However, there are times when pipe fittings, pipe thickness, the nature of pipe materials, soil conditions, and/or pipe configurations, can slow or even stop the static pipe bursting process. In other words, the capacity of the hydraulic system is challenged or even exceeded. This can result in unacceptably slow bursting rates, or even in the need for further excavation to remove the offending obstruction, replacement of the cable or puller with more robust apparatus, or can even result in cable breaking.
Accordingly, there is a need for improving the static pipe bursting process and apparatus to efficiently increase their capability to deal with larger pipe, obstructions, and related issues at a low cost in money, size, and power.